The present invention relates to a hydronic heating system. More specifically, the invention relates to a control unit and a method of operating a control unit that controls the temperature of water distributed in a radiant loop of a hydronic heating system having a primary loop, a secondary, radiant loop and a boiler.
Oil and gas fired boilers have long been used to supply hot water for hydronic heating in a building. Conventional hydronic heating systems circulate a supply of heated water through a series of heat exchangers positioned in the individual rooms of the building.
A simple hydronic heating system consists of a single boiler and a circulating pump that are controlled by a control unit that responds to a demand for heat from a single room thermostat. The single room thermostat only allows one temperature to be specified by the homeowner. The temperature in the vicinity of the thermostat will be controlled to the desired level, but in other parts of the house, the temperature can vary widely due to inadequate air distribution, solar radiation entering through outside windows, outside wind, and heat generated by people and other appliances. In response to these problems and the desire for greater comfort and flexibility, zoned heating systems have been developed.
A zoned heating system divides a building into a series of heating zones, each of which has an individual thermostat and flow control means, such as a valve. A zoned heating system is advantageous in that the user can selectively set the temperature in the different heating zones, which results in increased energy savings since the user is able to divert an increased amount of heat into the occupied rooms, while decreasing the amount of heat into rooms that may not be occupied.
In a hydronic heating system incorporating separate heating zones, the heating system often includes a primary loop and a secondary, radiant loop. The primary loop provides a path for heated water leaving the boiler to recirculate through the boiler without passing through any of the heating zones. Thus, the water flowing in the primary loop returns to the boiler at nearly the same temperature as it left the boiler, since little heat is dissipated from the water within the primary loop itself.
The secondary, radiant loop includes the series of heat exchangers positioned in the individual heating zones. Heated water from the boiler is circulated through the heat exchangers in the individual heating zones, such that heat is dissipated from the heated water to provide the required heating for the particular heating zone.
In many hydronic heating systems including both a primary loop and a radiant loop, a modulating valve is positioned between the primary and radiant loops to divert the flow of heated water between the two loops as required. Typically, a mixing controller is utilized to operate the modulating valve in a manner to control the temperature of water in the secondary, radiant loop.
In recent years, several advances have been made to increase the operating efficiencies of hydronic heating systems. For instance, mixing controllers that modify the water temperature in the secondary, radiant loop based on the outdoor air temperature have been developed. For example, one manufacturer offers a mixing controller that adjusts the temperature of the heated supply of water based on the outdoor temperature. Typically, systems like this include a ratio adjustment mechanism that allows the user to adjust the heating curve such that the temperature of the water is raised or lowered a selected amount for each degree of change in the outdoor air temperature. This type of adjustment for the supply water temperature based on the outdoor air temperature is called reset control. As discussed, most mixing controllers generate a heating curve that is adjustable by a ratio adjustment switch, such that the control can be set to adjust the supply water temperature based on the amount of change in the outdoor air temperature. However, this type of adjustment for the reset curve requires training and an understanding of the relationship between the supply water temperature and changing of the reset ratio, since the relationship may not be readily apparent.
In many types of boilers, heated flue gases generated by combustion within the boiler are fed into a heat exchanger surrounded by the supply of water to be heated. The heated flue gases transfer their heat through the heat exchanger and into the supply of stored water, thereby increasing the temperature of the water within the boiler. Often, the heat exchanger is manufactured from a material that is subject to corrosion, such as cast iron. In a hydronic heating system that includes heating zones that rapidly dissipate heat from the heated water circulated through the secondary, radiant loop, the water returning to the boiler from the secondary, radiant loop often is well below the temperature of the water leaving the boiler. If the return water temperature falls below a certain threshold in relationship to the temperature of the heated water, the cold water in the boiler may cause condensation within the heat exchanger, thus creating a possibility for corrosion within the heat exchanger. Some heat exchangers may also be susceptible to cracking if a large volume of cold water is introduced into an already hot heat exchanger (a condition known as thermal shock).
Therefore, it can be appreciated that a hydronic heating system controller that provides accurate and easily understandable reset control, as well as protection against possible condensation and thermal shock in the heat exchanger, would be a desirable improvement in the field of hydronic heating.